After nearly 1 hour of trying to decipher the problem why when trying to connect to the Wifi Service on the train and getting an error, I have finally found the issue.
The error you get is that the page re-directs to itself in a never-ending loop. This happens regardless of whether using Windows or MacOs.
If you have configured a different DNS server (e.g. 184.108.40.206) then you cannot connect to the NI Railways Wifi, you have to use their DNS servers, and set them to automatic.
Phew – finally online!